Differential pressure switch



Dec. 2, 1947. A E, BAAK 2,431,674

DIFFERENTIAL PRESSURE SWITCH Filed April 5, 1941 f/ f ////////////////ygi i lNvENToR Alloczxf' EoLcLk ATTORNEY Patented Dec. 2, 1947DIFFERENTIAL PRESSURE SWITCH Albert E. Baak, Minneapolis, Minn.,assignor to Minneapolis-Honeywell Regulator Company, Minneapolis, Minn.,a corporation of Delaware Application April 3, 1941, Serial No. 386,581

'l Claims. (Cl. 20G-82) This invention relates to automatic switcheswhich are actuated in accordance with the difference in pressure betweentwo iluids.

One of the main objects of the invention is to design a diierentialpressure switch which is particularly adapted for use on airplanes wherethe switches are subjected alternately to the very low atmosphericpressures encountered at high a1- titudes and the relatively highatmospheric pressures encountered on the ground. Itis customary to mountthe switch contacts in a sealed chamber to prevent dirt and moisturefrom the air from dirtying or rusting the contacts. It has been foundhowever that where this sealed chamber is subjected alternately to highand low external atmospheric pressures there is a tendency for thechambery to breathe slightly with the result .that moisture gets intothe chamber and rusts the contacts, It is therefore one of the objectsofthe invention to design a switch in Awhich outside air cannot get atthe contacts to corrode them.

Another object of the invention is to provide a pressure diierentialswitchv inv which the coritacts are mounted directly in the fluid, thepressure of which actuates the switch. l

A further object of the invention is to provide a sleeve with a closelyfitting rod respondingy to the difference in pressure at each end of thesleeve, the rod actuating a switch in a pressure chamber at one endofthe sleeve;

A further Objectis toprovide a switch of the above type inWhichfthere'are two stationary contacts and a movable bridging contactoperated by the differential pressure l'responsive rod, the bridgingmember being provided with a press back in order to align itself withrespect to the two stationary contacts.

These and other objects will readily become apparent to those skilled inthe art if the following speciilcation is read in the light of theaccompanying drawings in which: v

Figure 1 `is a cross-section of my switch housing enclosing twostationary switch contacts, and

`Figure 2 is a cross-section of the single sta-l tionary contact switch,the switch housing being shown as rotated through" an angle of 90 withrespect to' Figure 1.

Referring now to Figure 1, the reference numeral I indicates a switchhousing having an open end II, a main bore I2,y a counterbore I3 havinga smaller diameter than the ymain bore, and a second counterbore I4having a snialler diameter than the ilrst counterbore. One endl of ahollow cylindrical sleeve I5 is iltted into the counterbore I4 and the lextends well within the counterbore I3. The inner end of the sleeve I5is provided with an opening I1 which registers with a passageway I8extending through the side of the switch housing III. Passageway I8communicates with an annular slot I9 in the face portion 20 of theswitch housing I0. A second passageway 22 connects the chamber formed bythe counterbore I3 with an opening 23 in the face 20 of the switchhousing I0. yThe annular slot I9 and the opening 23 are designed forconnection with a pressure uid conducting member which may abut the face20. This member will conduct a uid, such as oil under pressure, to theannular slot I9 which pressure will be communicated through thepassageway I8 and opening I1 to the inner end of the sleeve I5. Anotherfluid, such as oil, will be conducted to the opening 23 and hencethrough passageway 22 to the chamber formed by the counterbore I3. Theslot I9 is made of annular formation so that the iluid conducting membermay be mounted in any rotative position withl respect to the switchhousing I0 and still makei the proper connections with the twopassageways I8 and 22.

kA diiferential pressure responsive member in the form of'a rod 25closely fits the interior of the sleeve I5 and is therefore responsiveto the difference in the pressures existing in the chamber formed by theinner end of sleeve I5 and the chamber formed by the counterbore I3 andmain bore I2. The rod 25 is provided with a reduced stem 26 which in oneposition of the rod will abut the inner end of the counterbore I3 toprevent the rod from closing oi the opening II.

At its outer end the rod 25 carries a cup 21 by any suitable connectionsuch as shown at 28. The cup is hollow and is provided with an enlargedhead 29 which forms a shoulder 30. Within the cup is a press back spring3|.which engages a press back button 32 which is made of some suitabletmolded insulating material such as Bakelite. The press back button 32has a rounded head 33 which is received in an indentation 1n a Steeldisc 34 which in turn backs up an electrical bridging member in the formof a disc made of some suitable electrically conducting material, suchas silver, indicated at 35. These two discs 34 and 35 aremounted in anannular holding member made of suitable insulating material, such asBakelite, and indicated at 36. In order tjo hold these parts inrassembled relation, the end of the enlarged head 29 vof the cup 2I.isturned over to form a lip as indicated at 3l. It will vbe noted that the2 other end of the sleeve I5 prevent the disc 34 from ever makingcontact with the cup 21 and groundingthe switch. When there is noexternal pressure applied to the discs 34 and 35, the press back spring3I holds them in the position shown in Figure 1 with relation to the cup21 and holdingmember 35. f

Surrounding the sleeve I5 and abutting the inner end of the counterboreI3 is a spring retaining washer 40 against which one end of the mainspring 4I rests. The other end of the main spring 4I surrounds the shankof the cup 21 and rests against a plurality of adjusting washers 42which in turn are seated against the outer face of the shoulder 3U, Thefunction of the spring 4I is to oppose the action of the rod 25 inmoving the contact head or cup 21 to the right as seen in Figure 1. Theeffective length and therefore the tension of the spring 4I may beadjusted by removing or inserting additional washers between the end ofthe spring 4I and the contact cup 21.

The open end II of the switch housing I is M closed by the flangedmember 45 which is suitably attached to the housing I0 by means of thescrews 46 which tighten the annular flange on the member 45 against agasket 41 locatedvbetween it and the open end II of the switch housing I0. The closing member 45 is-hollow and carries a pair of stationarycontacts 48 which are sealed into and insulated from said closing member45 and from each other by means of a suitable insulating material, suchas Bakelite 49, which is molded into the hollow portion of the closingmember 45. These two contacts 48 are provided with suitably roundedheads 50 which extend beyond the closing member 45 and into the chamberformed by the main bore I2. These contacts have elongated shanks I whichextend through the Bakelite 49 and into a chambered portion 52 of theclosing member 45. The charnbered portion 52 may be externally threadedas 'rat 53. The circuit to be controlled bythis switch is connected tothe shanks 5I of the two contacts 48 by a connector (not shown) engagingthe external threads on the chambered portion 52.

It will be seen that the chamber formed by the inner end of the sleeveI5 is connected to one source of fluid pressure whereas the chamberformed vby the two bores I2 and I3 is connected to another source offluid pressure. inasmuch as the rod 25 is subjected at one end to thepressure in the rst mentioned chamber and at the other end to thepressure in the second mentioned chamber, it will respond to thisdin/erential pressure. In the speciiic embodimentsherein disclosed, thefluid, whose differential-pressure is being measured, is oil, and thehigher pressure is within the chamber formed by the bores I2 and I3,which will hereinafter be referred to as the high pressure chamber. Anincrease in the difference in pressure between the high and low pressurechambers will therefore force the rod 25 to the right and separate thesilver bridging contact 35 from the two stationary contact heads 50.When the pressure diierential is reduced, the spring 4I will force therod 25 to the left and the bridging member 35 will again close thecircuit between the two stationary contacts 5U. The slight amount ofpress back offered by the -press back button 32 and spring 3I permitsthe bridging member 35 to align itself properly with the contacts 50,thereby assuring that both of these contacts will always be engaged bythe bridging member on movement thereof to the left by the rod 25.

By mounting the two contacts and the bridging member 35 directly in theoil, whose diierential pressure is being measured, it is assured that noair will be able to get at the contacts and corrode them, The pressurewithin both the high and low pressure chambers will be above thatof theexternal atmosphere and therefore there will be no breathing effect ofthis chamber as the switching device passes from low to high altitudesand vice versa.

Figure 2 shows a slight modification of the invention disclosed inFigure 1 wherein only a single stationary contact is used. The form ofswitch housing used in Figure ,2 is exactly the same as that used inFigure 1, and in order t0 show its form more clearly the section ofFigure 2 is taken looking directly down from the top of the sheet on thehousing shown in Figure 1.

The sleeve I5, rod 25, opening I1, passageways I8 and 22, as well as theannular slot I9 and opening 23, are exactly the same in Figure 2 as inFigure 1. Inasmuch as there is but a single stationary contact in themodication shown in Figure 2, there is no need for the silver contactbridging member 35 to align itself and therefore the press back has beenremoved. A metal washer 36 has been substituted in order that thecontact 35 may be well grounded through the cup 21, rod 25, sleeve I5 tothe housing I0 or from the cup 21, through the washers 42 and coilspring 4I.

The stationary contact in this modication is in exactly the same form asthe two contacts 48 of Figure 1 except that it is located directly inthe center of the closing member 45, Again this contact is sealed intothe closing member 45 by means of Bakelite 49 and is provided with aninner rounded head 50 which extends into the high pressure chamber andis also provided with a shank 5I which extends through the closingmember 45 and into the hoilowed out portion 52 where it may be connectedin an external circuit. The operation of the switch of this modificationis exactly the same as the operation of the modication disclosed inFigure 1 and itwill not be described in further detail. y

As various modiiications of this invention will occur to those who areskilled in the art, I intend to be limited only by the scope of theappended claims and not in accordance with the specific embodimentsdisclosed.

I claim as my invention:

l. A housing having a longitudinal bore extending partially therethroughto form a chamber having an open end and a closed end, a counterbore ofsmaller diameter than said main bore in the closed end of said housing,a hollow sleeve seated in said counterbore and extending into said mainbore, a rod slidably mounted in and closely fitting said sleeve, apassageway connected at one end to the inner end of said counterborebeneath said rod and adapted to connect at its other to a iluid underpressure, the space between the inner end of said rod and the inner endof said counterbore forming a pressure chamber, a second passagewayconnecting at one end to said main bore and adapted to connect at itsother end to another fluid under pressure, means for closing the openend of said housing to form a second pressure chamber, said rod beingmovable by the difference in pressure existing in said two chambers,stationary contact means in said second pressure chamber, a contactmoved by said rod into and out ofk engagement with said stationarycontact ,-means, and a coil spring determining'the pressure differentialat which said contacts shall beengaged and disengaged, said springsurrounding said sleeve and seating at the inner end of said main bore.

2. A housing having a longitudinal bore extending partially therethroughto form a chamber having an open end and a closed end,l a counterbore ofsmaller diameter than said main bore in the closed end of saidhousing, ahollow sleeve seated insaid counterbore and extending into said mainbore, a rod slidably mounted in and closely fitting said sleeve, apassageway connected at one end to the inner end of said counterborebeneath said rod and adapted to connect at its other to a iiuid underpressure, the space' between the inner end of said rod and the inner endof said counterbore forming a pressure chamber,a second passagewayconnecting at one end to said main bore and adapted to connect at itsother end to another fluid under pressure, means for closing the openend of said housing to form a second pressure chamber, said rod beingmovable by the difference in pressure existing in said two chambers,stationary contact means in said second chamber, a contact head carriedby said rod, a contact on said head moved by said rod into and out ofengagement with said stationary contact means, and a coil springresisting movement of said head by said rod, said spring surroundingsaid sleeve and being seated at the inner end of said main bore, and oneor more washers between the end of said spring and said contact head,the lnumber of washers predetermining the tension of said spring andhence thepressure difference at which said piston'will actuate saidcontacts.

3. A housing having a longitudinal bore extending partially therethroughto form a chamber having an open end and a closed end, a counterbore ofsmaller diameter than said main bore in the closed end of said housing,a hollow sleeve seated in .said counterbore and extending into said mainbore, a rod slidably mounted in and closely fitting said sleeve, apassageway connected at one end to the inner end of said counterborebeneath said rod and adapted to connect at its other to a uid underpressure, the space between the inner end of said rod and the inner endof said counterbore forming a pressure chamber, a second passagewayconnecting at one end to said main bore and adapted to connect at itsother end to another fluid under pressure, means for closing the openend of said housing to form a second pressure chamber, said rod beingmovable by the difference in pressure existing in said two chambers, apair of stationary contacts extending through said closing means andinto said second chamber, said contacts being sealed to said closingmember and insulated from each other, a contact head carried. by saidrod, an electrical conducting contact bridge moved by said rod into andout of bridging relationship with said stationary contacts, press backmeans associated with said bridge to provide for an aligning movementwhereby it will always engage both of said stationary contacts, and acoil spring resisting movement of said contact head by said rod, saidspring surrounding said sleeve and seating at the inner end of said mainbore.

4. A pressure operated switch for use in airplanes, a switch chamber, apair of relatively movable switch contacts mounted therein, pressureresponsive means forming a movable wall of said chamber for actuatingsaid switch contacts, an velectrically non-conductive fluid lling saidchamber at superatmospheric pressure, means connecting said chamber witha source of variable pressure whereby variations in the pressure of saiduid cause movement of said ,pressure responsive meansto actuate saidswitch contacts, said uid preventing the leakage of air into saidchamber as said switch is moved from an area of low barometric pressureto one of high barometric pressure, said uid being of a type which willnot rust or otherwise harm said switch contacts.

5. A housing having a longitudinal bore extending partially therethroughto form a chamber having an open end and a closed end, a counterbore ofsmaller diameter than said main bore in the closed endof said housing, ahollow sleeve seated in said counterbore and extending into said mainbore, a rod mounted in and` closely` fitting said sleeve, a passagewayconnected at one l end to the inner end of said counterbore beneath saidrod and adapted to connect at its other to a fluid under pressure, thespace between the inner end of said rod and the inner` end of saidcounterbore forming a pressure chamber, a second passageway connectingat one end to said main bore and adapted to connect at its other end toanother fluid under pressure, means for closing the open end of saidhousing to form a second pressure chamber, said rod being movable by thedifference in pressure existing in said two chambers, stationary contactmeans in one ofsaid chambers, and contact means moved by said rod intoand out of engagement with said stationary contact means.

6. An aircraft differential pressure switch for responding to thedifferential in two fluid pressures which are always maintained aboveatmosy V pheric; comprising; in combination; means forming a relativelylarge pressure chamber, a smaller pressure chamber and a connecting boretherebetween having a relatively small diameter in respect to said largepressure chamber and a relatively long length in relation to its owndiameter, one of said chambers having an opening therein; a diierentialpressure responsive assembly including a head larger than said borelocated in said large pressure chamber and a cylindrical member ofreduced size extending therefrom and into said bore, said cylindricalmember having a diameter that closely fits said bore and a length suchthat it at least substantially fills said bore;

a member closing said opening and sealing said chamber from theatmosphere; a movable contact carried by said assembly and located inone of said pressure chambers; a cooperating stationary contact sealedinto and extending through said closing member; and a spring bearingupon said enlarged head biasing said assembly in a direction to closesaid switch and forming a part of the electrical circuit to said movablecontact; whereby when said pressure chambers are connected to sources ofsuper-atmospheric iiuid pressure in an aircraft, the contacts arecontinuously subjected to a iiuid pressure sufficiently high to preventthe infiltration of moisture regardless of the changes in altitude oratmospheric conditions to which said switch is subjected.

7. A differential pressure switch for aircraft comprising incombination, a body forming a pair of chambers of differentcross-sectional area, each of said chambers having an opening thereinfor connection to a source of super-atmospheric pressure and one of saidchambers having a iurther opening therein, a member closing said furtheropening of said one chamber and sealing it from the atmosphere.electrical conducting means extending through said closing member andinto one of said chambers, said electrical conducting means being sealedinto said member, a movable contact in one of said chambers for engagingsaid electrical conducting means to permit the ow of currenttherethrough, spring means blasing said movable contact into engagementwith said electrical conducting means, and means responsive to thedifferential pressure in said two chambers for moving said movablecontact away from said electrical conducting means when saiddifferential pressure increases to a predetermined value.

ALBERT E. BAAK.

REFERENCES CITED The `following references are of record in the ille ofthis patent:

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